The present invention relates to a multi-stage process for preparing ethylene homo and copolymers having broad molecular weight distributions and low content of soluble fractions, said process comprising a first polymerization stage (A) in the presence of Ti or V catalysts, a treatment stage (B) wherein the Ti or V catalyst is deactivated and a specific metallocene is supported on the polymer produced in stage (A), optionally in the presence of a suitable cocatalyst, and a final polymerization stage (C).
The invention also relates to the broad molecular weight distribution polyethylene obtainable by the above process.
Multistage processes for the polymerization of olefins, carried out in two or more reactors, are well known in the state of the art and are particularly interesting in industrial practice. Said processes are generally carried out using the same catalyst in the various stages/reactors, by utilizing tandem reactors operated in series: the product obtained in one reactor is usually discharged and sent directly to the following stage/reactor without altering the nature of the catalyst.
Broad or multi-modal molecular weight distribution (MWD) polyethylenes are commonly prepared by employing a multi-reactor process, wherein ethylene is polymerized in various reactors containing the same catalyst but in the presence of different concentrations of hydrogen, as molecular weight regulator. The thus obtained polymers contain a high molecular weight (HMW) fraction and a low molecular weight (LMW) fraction, therefore showing a broad total molecular weight distribution (MWD=Mw/Mn).
Nevertheless, said polymers present various drawbacks due to the LMW fraction obtained in the reactor with the higher amount of hydrogen; said fraction, having itself a broad MWD, contains undesired soluble products of very low molecular weight, which impair the mechanical properties of the final polymer and do not allow its use in medical and alimentary fields.
Polyolefins having broader MWD are products of notable commercial value, since they show high workability due to the HMW fractions and, at the same time, they provide excellent mechanical properties, due to the LMW fractions. Said polyolefins have been produced even in a single polymerization process, by employing two distinct and separate catalysts in the same reactor each producing a polyolefin having a different MWD. For instance, the European patent application EP 0 128 045 describe the use of a bimetallic catalyst system, i.e. a catalysts system comprising two or more metallocenes, each one having different propagation and termination rate constants, and alumoxanes; the polymers obtained by using said system in a single polymerization process have a broader multimodal molecular weight distribution than using a single metallocene, that usually produces polyethylenes with Mw/Mn of from 2 to 4; nevertheless, the obtained polymers have Mw/Mn lower than 8.
Also the European patent applications EP 0 619 325 and EP 0 705 851 describe the production of multimodal MWD polyolefin from a single polymerization process in the presence of a catalytic system comprising two different metallocenes, wherein at least one of the metallocenes is bridged, generally in the presence of hydrogen as MW regulator. The amount and productivity of the two metallocenes used is varied to control the relative amounts of the HMW and LMW fractions in the final polymer; nevertheless, these parameters are difficult to control and lead to non-homogeneous final products, not uniform in size, having unpredictable properties.
The European patent application EP 0 673 950 describes the preparation of polyethylene having a broad MWD in a gas phase reactor, in the presence of a prepolymer containing a Ti/Zr bimetallic catalyst and of hydrogen. The thus obtained polymers show very low Mn values and are obtained with low activities.
The European patent application EP 0 514 594 describes the production of polymers having multimodal molecular weight distribution in a single polymerization reactor wherein the catalytic system comprises a Mg support, a Ti or V based component, a zirconocene and suitable cocatalysts. Said catalytic system allows bimodal MWD polymers to be obtained; more specifically, the LMW polymer fraction is produced by the metallocene component, while the HMW fraction is due to the Ti or V component.
The several known methods employing mixtures of catalytic systems in one stage polymerization, as described above, have many drawbacks; in particular, the catalyst feed rate is difficult to control and the polymer particles produced are not uniform in size; segregation of the polymer during storage and transfer usually produce non-homogeneous products.
In order to overcome the above difficulties, various solutions have been proposed. The International patent application WO 96/07478 describe a single polymerization process carried out in the presence of a catalyst comprising two different transition metal compounds, each having different hydrogen response characteristics. The proportion of different weight fractions in broad or bimodal molecular weight distribution is controlled by adding a further amount of one of the two components of the bimetallic catalyst. Another solution is that disclosed in the International patent application WO 96/09328, wherein water and/or carbon dioxide are co-fed to the polymerization reactor, containing a bimetallic catalyst, at levels necessary to modify the weight fractions of the HMW and LMW components, thus achieving a target molecular weight distribution. However, the above solutions are still not satisfactory.
The International patent application WO 96/11218, in the name of the same Applicant, describes a multi-stage process for olefin polymerization, in particular for the preparation of heterophase copolymers of propylene, comprising a first stage wherein a first olefin polymer is prepared in the presence of titanium or vanadium catalysts; a second stage of deactivation of the catalyst used in the first stage; and a third stage of polymerization of one or more olefins in the presence of the polymer of the first stage and of a complex of a transition metal M (M being Ti, Zr, V or Hf) containing at least one M-xcfx80 bond and/or of their reaction products. Said process proved to be particularly useful in the production of polymers with broad MWD, even if the metallocenes used did not allow a sufficiently low Mw to be obtained.
Therefore, it is desirable to provide a novel, high activity process for the production of broad MWD polyethylenes having low xylene soluble fractions, without incurring in the disadvantages of the above discussed prior art.
It has now been found that broad MWD ethylene homo and copolymers can be obtained through a multi-stage process, by using a new class of zirconocenes which are unexpectedly able to produce, in high yields, ethylene polymers having fairly low molecular weights and narrow MWD, without necessitating the use of molecular weight regulators, such as hydrogen.
More specifically, the present invention provides a multi-stage process for the polymerization of ethylene, optionally in the presence of one or more (x-olefins, comprising from 3 to 10 carbon atoms, to produce a polymer having a broad MWD; said process comprises the following stages:
(A) polymerizing ethylene, and optionally said xcex1-olefin, in one or more reactors, in the presence of a catalyst comprising the reaction product between:
(i) a solid component comprising a compound of a transition metal MI selected from Ti and V, not containing MI-xcfx80 bonds, and a halide of Mg in active form, optionally comprising an electron-donor compound (internal donor);
(ii) an alkyl-Al compound and optionally an electron-donor compound (external donor);
in order to produce an ethylene homo or copolymer;
(B) contacting the product obtained in stage (A), in any order whatever, with:
(a) a compound capable of deactivating the catalyst of stage (A);
(b) a zirconocene compound of formula (1): 
wherein R1, R2, R3 and R4, the same or different from each other, are selected from the group consisting of hydrogen, linear or branched, saturated or unsaturated, C1-C20 alkyl, C3-C20 cycloalkyl, C6-C20 aryl, C7-C20 alkylaryl and C7-C20 arylalkyl radicals, optionally containing one or more Si or Ge atoms, or wherein two substituents of R1, R2, R3 and R4 form a ring having from 4 to 8 carbon atoms;
R5 and R6, the same or different from each other, are selected from the group consisting of linear or branched, saturated or unsaturated C1-C20 alkyl, C3-C20 cycloalkyl, C6-C20 aryl, C7-C20 alkylaryl and C7-C20 arylalkyl radicals, optionally containing one or more Si or Ge atoms, or wherein one pair of vicinal R6 substituents of the same indenyl group forms a ring having from 4 to 8 carbon atoms;
m is an integer ranging from 0 to 2; n is an integer ranging from 0 to 4;
the groups X, the same or different from each other, are hydrogen, halogen, xe2x80x94R, xe2x80x94OR, xe2x80x94SR, xe2x80x94NR2 or xe2x80x94PR2, wherein R is selected from the group consisting of linear or branched, saturated or unsaturated, C1-C20 alkyl, C3-C20 cycloalkyl, C6-C20 aryl, C7-C20 alkylaryl and C7-C20 arylalkyl radicals, optionally containing one or more Si or Ge atoms; and
(c) optionally an activating cocatalyst;
(C) polymerizing ethylene and optionally said xcex1-olefin, in one or more reactors, in the presence of the polymer obtained from stage (B).
Another object of the present invention are new polyethylenes obtainable according to the above multi-stage process, having broad MWD, average molecular weights of industrial interest and showing very low xylene soluble fractions. More specifically, the polyethylenes according to the present invention have the following characteristics:
1) intrinsic viscosity (I.V.) ranging from 0.5 to 6 dl/g, preferably from 1 to 4, and more preferably from 1.5 to 3 dl/g;
2) Mw/Mn (i.e. MWD) greater than 8, preferably  greater than 10, and more preferably  greater than 11;
3) cold xylene soluble fraction XS less than 1.2% wt., preferably  less than 1% wt., and more preferably  less than 0.8 % wt.
The broad MWD polyethylenes of the present invention have good processability, while maintaining good mechanical properties.
The multi-stage process for the polymerization of ethylene and optionally xcex1-olefins, and the broad MWD polyethylenes thus obtainable, according to the present invention, will be better described in the following detailed description.
Stages (A)-(C) of the process of the invention are preferably carried out according to the operating conditions given in the cited International patent application WO 96/11218.
The first polymerization stage (A) allows the obtainment of a polymer fraction having high molecular weight, by using a conventional Ti or V-based catalytic system. Said stage (A) can be carried out in liquid phase or in gas phase, working in one or more reactors. The liquid phase can consist of an inert hydrocarbon solvent (suspension process), optionally in the presence of one or more xcex1-olefins, comprising from 3 to 10 carbon atoms. Gas-phase polymerization can be carried out using the known fluidized-bed technique, according to standard procedures, or working in conditions in which the bed is mechanically stirred, optionally in the presence of one or more of said xcex1-olefins.
The catalyst used in the first stage of polymerization (A) comprises the product of the reaction between.
(i) a solid component comprising a compound of a transition metal MI selected from Ti and V, not containing MI-xcfx80 bonds, supported on a halide of magnesium in active form, optionally comprising an electron-donor compound (internal donor);
(ii) an alkyl-Al compound and optionally an electron-donor compound (external donor).
Said halides of magnesium in active form, preferably MgCl2, used as a support for Ziegler-Natta catalysts, are widely known from the patent literature. U.S. Pat. No. 4,298,718 and U.S. Pat. No. 4,495,338 first described the use of these compounds in Ziegler-Natta catalysis. It is well known that the halides of magnesium in active form, used as support or co-support in components of catalysts for the polymerization of olefins, are characterized by X-ray spectra in which the most intense diffraction line that appears in the spectrum of the non-active halide is diminished in intensity and is replaced by a halo whose maximum intensity is shifted towards lower angles compared with that of the most intense line.
The compound of the transition metal MI is selected preferably from the group consisting of halides of titanium, halogen-alcoholates of titanium, VCl3, VCl4, VOCl3 and halogen-alcoholates of vanadium.
Among the titanium compounds, the preferred are TiCl4, TiCl3 and the halogen-alcoholates of formula Ti(OR1)rXs, wherein R1 is a C1-C12 hydrocarbon radical, or is a group xe2x80x94COR1; X is halogen and (r+s) is equal to the oxidation state of Ti.
The catalytic component (i) is advantageously used in the form of spheroidal particles with mean diameter ranging from about 10 and 150 xcexcm. Suitable methods for the preparation of said components in spherical form are reported for instance in European patent applications EP 0 395 083, EP 0 553 805, EP 0 553 806.
The internal donor optionally present in the catalytic component (i) can be an ether, an ester, preferably an ester of a polycarboxylic acid, an amine, a ketone; preferably, said internal donor is a 1-3,diether of the type described in European patent applications EP 0 361 493, EP 0 361 494, EP 0 362 705 and EP 0 451 645.
The alkyl-Al compound (ii) is preferably a trialkyl aluminum compound, such as triethyl-Al, triisobutyl-Al, tri-n-butyl-Al, tri-n-hexyl-Al, tri-n-octyl-Al and triisooctyl-Al. It is also possible to use mixtures of trialkyl-Al""s with alkyl-Al halides, alkyl-Al hydrides or alkyl-Al sesquichlorides, such as AlEt2Cl and Al2Et3Cl3.
The external donor present in the catalytic component (ii) can be the same or different from the internal donor. When the internal donor is an ester of a polycarboxylic acid, such as a phthalate, the external donor is preferably a silicon compound of formula Rxe2x80x3Rxe2x80x3Si(ORxe2x80x3)2, wherein the groups R11, the same or different from each other, are C1-C18 alkyl, cycloalkyl or aryl radicals. Particularly advantageous examples of such silanes are methylcyclohexyldimethoxysilane, diphenyldimethoxysilane, methyl-t-butyldimethoxysilane and dicyclopentyldimethoxysilane.
The polymer obtained from the polymerization stage (A) has preferably a porosity, expressed as percentage of voids, higher than 5%, preferably higher than 10%, and more preferably higher than 15%. Said polymer is preferably characterized by macroporosity, wherein more than 40% of the porosity of the said polymers is due to pores with diameter higher than 10,000 xc3x85; more preferably, more than 90% of the porosity is due to pores with diameter higher than 10,000 xc3x85. The porosity, expressed as percentage of voids, and the distribution of pore radius are determined by absorption of mercury under pressure, according to the procedure reported in WO 96/11218.
The amount of polymer produced in the first stage of polymerization (A) is generally greater than 1000 g/g of solid component, preferably greater than 2000 g/g, more preferably greater than 3000 g/g. The amount of polymer produced in polymerization stage (A) is preferably between 10 and 90% by weight relative to the total amount of polymer produced in stages (A) and (C) and more preferably is between 20 and 80%. Stage (B) envisages, in any order whatever, the deactivation of the titanium-based catalyst used in stage. (A) and the supportation of the above specified zirconocene (I), and optionally of a suitable cocatalyst, on the polymer obtained in stage (A).
According to a preferred embodiment of the multi-stage process of the invention, in stage (B), the product obtained from stage (A) is first contacted with said compound (a) capable of deactivating the catalyst used in stage (A); then, the deactivated product thus obtained is contacted, in any order whatever, with said zirconocene compound (b) and optionally said activating cocatalyst (c). Preferably, after the treatment with the deactivating compound (a), any excess of the deactivating compound is removed, according to procedures known in the state of the art.
More specifically, stage (B)(a) comprises bringing into contact the polymer produced in polymerization stage (A) with compounds that are able to deactivate the catalyst used in said stage (A). The deactivation stage (B)(a) is necessary so as to avoid that the titanium based catalyst used in stage (A) is active in the polymerization stage (C), that would lead to the production of a too high molecular weight polymer, which would be unprocessable.
Stage (B)(b) comprises bringing into contact the product obtained in (a) with a zirconocene of formula (I), preferably a solution of the zirconocene of formula (I) in hydrocarbon solvents (benzene, toluene, heptane, hexane, liquid propane and the like), in order to support said zirconocene on the polymer obtained from stage (A).
Stage (B)(c), which can be carried out before, after or at the same time as (B)(b), comprises bringing into contact the product obtained in (a) with a suitable cocatalyst. Examples of compounds that can be used in treatment stage (a) can be selected from the group consisting of compounds having the general formula Rxe2x80x2xe2x80x3yxe2x88x921XH, wherein Rxe2x80x2xe2x80x3 is hydrogen or a C1-C10 hydrocarbon group; X is O, N, or S; and y is the oxidation state of X. Non-limiting examples of such compounds are represented by alcohols, thioalcohols, mono- and di-alkylamines, NH3, H2O and H2S. Preferred compounds are those in which X is O and particularly preferred is water.
Other examples of compounds that can be used in treatment stage (B)(a) are CO, COS, CS2, CO2, O2 and acetylenic or allenic compounds. The molar ratio between the deactivating compound and the compound of the transition metal MI should preferably be such as to ensure substantial deactivation of the catalyst of stage (A). The value of this ratio is preferably greater than 50, more preferably greater than 150 and in particular greater than 250.
Treatment (a), in which these deactivating compounds are brought into contact with the polymer produced in stage (A), can be effected in various ways. In one of these, the polymer is brought into contact, for a time ranging from 1 minute to some hours, with a hydrocarbon solvent that contains the deactivating compound in solution, suspension or dispersion. An example of dispersion of the deactivating compound in a hydrocarbon solvent is represented by humidified hexane. At the end of treatment (a), the liquid is removed and the polymer undergoes treatment (b).
In the zirconocenes of formula (I) used in stage (B)(b):
R1, R2, R3 and R4 are preferably selected from the group consisting of hydrogen, methyl, ethyl, propyl, phenyl and benzyl; more preferably, R1, R2, R3 and R4 are hydrogen and the bridging group of the 2-indenyls is ethylene;
R5 and R6 are preferably selected from the group consisting of methyl, ethyl, propyl, phenyl and benzyl;
the groups X are preferably selected from the group consisting of Cl, Br or methyl.
Non-limiting examples of zirconocene compounds belonging to the said class are:
1,2-ethylene-bis(2-indenyl)zirconium dichloride,
1,2-ethylene-bis(1,3-dimethyl-2-indenyl)zirconium dichloride,
rac- and meso-1,2-ethylene-bis(1-methyl-2-indenyl)zirconlum dichloride,
rac- and meso-1,2-ethylene-bis(-ethyl-2-indenyl)zirconium dichloride,
rac- and meso-1,2-ethylene-bis(4-phenyl-2-indenyl)zirconium dichloride and
rac- and meso-1,2-ethylene-bis(1-methyl4-phenyl-2-indenyl)zirconium dichloride.
The zirconocene compounds of formula (I) can be prepared by reaction of the corresponding ligands first with a compound capable of forming a delocalized anion on the cyclopentadienyl ring, and then with a compound of formula ZrZ4, wherein the substituents Z, the same or different from each other, are halogen; ZrCl4 is particularly preferred.
When, in the zirconocene of formula (I) one or more X are other than halogen, it is necessary to substitute one or more substituents Z of the zirconocene dihalide with one or more substituents X other than halogen. The substitution in the zirconocene halide of Z with X other than halogen can be carried out by standard procedures, known in the state of the art, for example by reacting the zirconocene halide with alkylmagnesium halides (Grignard reagents) or with alkyllithium compounds.
The ligands useful to prepare the zirconocenes of the present invention can be synthesized by different procedures, and preferably as described in the European patent appl. no. 98200728.8 in the name of the same Applicant.
In stage (B)(c) of the process of the invention, suitable activating cocatalysts are organometallic aluminum compounds; particularly suitable are the organometallic aluminum compounds described in European patent application EP 0 575 875 (formula (II)) and those described in International patent application WO 96/02580 (formula (II)). Non-limiting examples of said organometallic aluminum compounds are: tris(methyl)aluminum, tris(isobutyl)aluminum, tris(isooctyl)aluminum bis(isobutyl)aluminum hydride, methyl-bis(isobutyl)aluminum, dimethyl(isobutyl)aluminum, tris(isohexyl)aluminum, tris(benzyl)aluminum, tris(tolyl)aluminum, tris(2,4,4-trimethylpentyl)aluminum, bis(2,4,4-trimethylpentyl)aluminum hydride, isobutyl-bis(2-phenyl-propyl)aluminum, diisobutyl-(2-phenyl-propyl)aluminum, isobutyl-bis(2,4,4-trimethyl-pentyl)aluminum and diisobutyl-(2,4,4-trimethyl-pentyl)aluminum.
The use of the above organometallic aluminum compounds is particularly advantageous when water is used, in stage (B)(a), as compound capable of deactivating the catalyst of stage (A).
The activating cocatalysts in stage (B)(c) of the process of the invention can even be the reaction product between water and one or more of the above-reported organometallic aluminum compounds.
Suitable activating cocatalysts in stage (B)(c) of the process according to the present invention are linear, branched or cyclic alumoxanes of formula: 
wherein the substituents R7, the same or different from each other, are linear or branched, saturated or unsaturated, C2-C20 alkyl, cycloalkyl, aryl, arylalkyl or alkylaryl radicals, or are groups xe2x80x94Oxe2x80x94Al(R7)2.
Examples of alumoxanes suitable for use according to the present invention are methylalumoxane (MAO), tetra-isobutyl-alumoxane (TIBAO), tetra-2,4,4-trimethylpentyl-alumoxane (TIOAO) and tetra-2-methyl-pentylalumoxane. Mixtures of different alumoxanes can also be used.
Mixtures of different organometallic aluminum compounds and/or alumoxanes can also be used.
Further suitable activating cocatalysts in stage (B)(c) of the process of the invention are compounds capable of forming an alkyl zirconocene cation. Non-limiting examples are the compounds of formula Y+Zxe2x88x92, wherein Y+ is a Bronsted acid, capable of donating a proton and of reacting irreversibly with a substituent X of the compound of formula (I), and Zxe2x88x92 is a compatible non-coordinating anion, capable of stabilizing the active catalytic species originating from the reaction of the two compounds, and sufficiently labile to be displaced by an olefinic substrate. The Zxe2x88x92 anion preferably comprises one or more boron atoms; more preferably, Zxe2x88x92 is an anion of formula BAr4(xe2x88x92), wherein the Ar substituents, the same or different from each other, are aryl radicals, such as phenyl, pentafluorophenyl and bis-(trifluoromethyl)phenyl. Tetrakis-pentafluorophenyl borate is particularly preferred. Moreover, compounds of formula BAr3 can be advantageously used.
Treatment stage (B)(b) is preferably carried out using the zirconocene in solutions of hydrocarbon solvents containing a dissolved activating cocatalyst, according to stage (B)(c), which is preferably an alkyl-aluminum compound, such as triisobutyl-aluminum (TIBA), tris(2,4,4-trinethyl-pentyl)aluminum (TIOA) and/or an aluminoxane, for example methylalumoxane (MAO), tetra-isobutylalumoxane (TIBAO), tetra(2,5-dimethylhexyl)-alumoxane and tetra(2,4,4-trimethyl-pentyl)alumoxane (TIOAO). The molar ratio of the alkyl-Al compound to the zirconocene is greater than 2 and is preferably between 5 and 1000. The stage (B)(b) can be carried out by suspending the polymer obtained from stage (B)(a) in hydrocarbon solvents, preferably propane, containing the dissolved zirconocene, and optionally an alkyl-Al compound and/or an aluminoxane (B)(c), generally working at temperature between 0 and 100xc2x0 C., preferably between 10 and 60xc2x0 C., and removing the solvent propane at the end of the treatment. Alternatively, the polymer obtained from (a) can be brought into contact, dry, with solutions of the zirconocene containing the minimum quantity of solvent for keeping the said compound in solution.
Stage (B) can be conveniently carried out in the gas phase in a loop reactor, in which the polymer produced in the first stage of polymerization is circulated by a stream of inert gas. Solutions of the deactivating compound and of the zirconocene are fed successively, for example using a sprayer, to the loop reactor in the gas phase, and a free-flowing product is obtained at the end of the treatment. Conveniently, before stage (b), the product is treated with compounds that are able to decontaminate the system, for example with alkyl-Al compounds.
The quantity of zirconocene, expressed as metal, contained in the product obtained from stage (B), can vary over a wide range depending on the zirconocene used and on the relative quantity of product that it is desired to produce in the various stages. Generally this quantity is between 1xc2x710xe2x88x927 and 5xc2x710xe2x88x923 g of Zr/g of product, preferably between 5xc2x710xe2x88x927 and 5xc2x710xe2x88x924, more preferably between 1xc2x710xe2x88x926 and 1xc2x710xe2x88x924.
The polymerization stage (C) can be carried out in liquid phase or in gas phase, working in one or more reactors, and it is directed to the synthesis of the low molecular weight polymer fraction. The liquid phase can consist of an inert hydrocarbon solvent (suspension process), optionally in the presence of one or more (xcex1-olefins, comprising from 3 to 10 carbon atoms. Gas-phase polymerization can be carried out in reactors with a fluidized bed or with a mechanically-stirred bed, optionally in the presence of one or more of said xcex1-olefins. During said stage (C), it is convenient to feed the polymerization reactor with an alkyl-Al compound selected from Al-trialkyls, wherein the alkyl groups contain from 1 to 12 carbon atoms, and linear or cyclic aluminoxane compounds containing xe2x80x94(R7)AlOxe2x80x94, wherein R7 has the meaning reported above, said aluminoxane compounds containing from 1 to 50 repeating units. As a general rule, said alkyl-Al compound is fed to polymerization stage (C) when treatment (c) in stage (B) is absent.
The process of the invention allows the preparation of ethylene polymers having broad MWD, average molecular weights of industrial interest and low xylene soluble fractions. Therefore, another object of the present invention are broad MWD polyethylenes obtainable by the multi-stage process according to the present invention, said polyethylenes having the following characteristics:
1) intrinsic viscosity (I.V.) ranging from 0.5 to 6 dl/g, preferably from 1 to 4 dl/g, and more preferably from 1.5 to 3 dl/g;
2) molecular weight distribution Mw/Mn  greater than 8, preferably  greater than 10, and more preferably  greater than 11;
3) cold xylene solubility XS less than 1.2% wt., preferably less than 1% wt., and more preferably  less than 0.8% wt.
Said polyethylenes, having a broad MWD, show very good processability properties and, at the same time, maintain excellent mechanical properties.